Dense composite membranes variation

A classic example of a composite membrane is that patented in 1916 by Snelling who used porous ceramics to support dense layers of palladium, 25 p,m thick [1]. Variations of his theme remain at the forefront of research [2, 3]. Examples include use of porous alumina, silica, perovskites, and stainless steel to support thin layers of Pd and its alloys [2, 3]. Snelling addressed issues of perforations and pinholes... 

IS measurements were performed to determine the membrane variations associated with (i) Dense and porous layers of a commercial RO membrane (ii) Different PEG concentrations in the top dense layer of a polyamide/polysulfone experimental membrane (iii) Hydrophobic character of one layer in a composite or multilayer structure (iv) Membrane matrix material modification and (v) Protein (BSA) fouling of a porous commercial membrane. The results obtained with other characterization techniques, such as morphological, chemical, and adsorption analyses, have validated the information obtained from the IS results. 

One version of a thin film composite membrane involves the coating of a very thin layer of a polymer onto a porous support such as a porous polymer membrane or hollow fiber. This constmction would be desirable if the dense layer is a very expensive specialty polymer or a rubbery material not offering the capability to support pressure. A variation of this method is an asymmetric thin film composite membrane produced by solution-coating a porous membrane and subjecting the thin film coating to a nonsolvent shortly after coating to yield an asymmetric layer. 

The thermodynamic description of the formation of mlcroporous systems by means of the phase diagrams, eis illustrated in Figures 1 to 3, is based on the assumption of thermodynamic equilibrium. It predicts under what conditions of temperature and composition a system will separate into two phases and the ratio of the two phases in the heterogeneous mixture. As related to the membrane formation procedure, the thermodynamic description predicts the overall porosity that will be obtained at specified states. However, no information is provided about the pore sizes, which are determined by the spatial distribution of the two phases. Equilibrium thermodynamics is not able to offer any explanation about structural variations within the membrane cross-section that is, whether the membrane has a symmetric or asymmetric structure or a dense skin at the surface. These... 

Figure 9.12 Variation of membrane/layer electrical resistance (a) and capacitance (b) for dense layer of composite HR95 membrane ( ), porous sublayer of composite HR95 membrane (O), porous PS-Uf membrane (O).

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